Experimental study of a fast plasma focus discharge operated in the range of tens of joules emitting neutrons

In this doctoral research, a small plasma focus discharge operated at tens of joules and kilo amperes has been experimentally studied. This device produces a coaxial discharge by an electrode arrange which consists of an inner electrode, a cylindrical insulator, and an outer electrode. The discharge is driven by a capacitive generator and it is operated at pressures of some millibars. Typical electrical parameters of the device are T/4∼150ns, 160 nF, 40 nH, 30-100J, 40-70kA. When Deuterium is used as filling gas, neutrons are produced by fusion reactions. The device, namely PF-50J, is one of the most extreme plasma focus reported in the literature to emit neutrons. The results of this work have demonstrated that the same dynamics found in larger machines (operated from kJ to MJ) is also found in this extremely low energy device which means that after the discharge is initiated, a plasma sheet is formed over the insulator. By the action of the Lorentz force, the plasma sheet moves axially. When the sheet reaches the top end of the inner electrode, it starts to implode radially to finally form a dense plasma column (pinch). Finally, the plasma column is disrupted 5-10ns after column formation. Shock piston velocities of the order of 104-105m/s were estimated from experiments for each discharge dynamical phase. Typical pinch densities of 1024-25m-3 were also observed in PF-50J. An interesting feature observed in this work is the formation of axial jet-like structures which appears at late times almost hundred nanoseconds after pinch disruption. These structures are composed by a metallic plasma which results from ablation of the inner electrode. The mechanism responsible for the formation of this kind of structure is still unknown, although evidence suggests that the phenomena is not related to the pinch formation process. One of the principal motivations of this thesis was to study the optimization for neutron emission in the PF-50J device. This plasma focus is expected to yield 103-104 neutrons per pulse when is operated with deuterium. Standard detection techniques, for example neutron activation detectors, present detection limits higher than 105 neutrons per pulse, so it was necessary to develop a measurement method for higher efficiency detectors based on the use moderated proportional counter tubes for the purpose of neutron optimization. In contrast to low rate continuous regime, this detectors show pulse piling up when irradiated with pulsed neutrons, which make impossible to count events with standard nuclear electronics. In this thesis, it is presented a detailed study of pulsed piling up statistics in the proportional counter and a complete characterization of the 3He tubes based detection system. Regarding that the net area of detector output signal is proportional to the number of detected events, a counting model was developed and thus a methodology for measurement of the neutron yield is proposed. Special emphasis was put on the sources of fluctuation that affects the measurement process, which are given by the counting statistics, piling up statistics, and background electrical noise. As a result, the methodology allows measurements uncertainties for pulsed D-D fusion neutrons lower than 30% in the range of Y ≥ 3x103n/shot, reducing in this way by almost two order of magnitude the detection limit of the state of art techniques. A theoretical study of the detection frequency or reproducibility for pulsed neutron sources is also presented. Comparisons with results in PF-50J show that theoretical models for reproducibility are reasonable high boundary estimations for the experimental values. The optimization study is presented in the last part of this work. Optimal conditions for neutron emission were experimentally obtained. An interesting finding is the observation of two regimes for pinch formation and neutron emission. The first regime is related to the pinch formation close to the first current maximum in the first quarter of period, while in the second one these phenomena are observed at higher pressures close to the first current minimum in the second quarter of period, and even also close to the second current maximum in the third quarter of period. This behavior is linked to the formation of multiple plasma sheets over the insulator and to the energy coupling between generator and load. In connection to the laws governing the optimization for neutron emission for a plasma focus device, boundary values linking global optimal configuration, namely, effective anode length and pressure with other operation parameters as anode radii and current peak are proposed. The validity of this scheme is particularly proved for the optimization of the PF-50J device. Attention was also put on some engineering issues. There was found that neutron emission is strongly affected by impurities generated by electrode ablation. In connection to that, there was observed that gas removal cycle and geometry of the focus chamber are of supreme importance for high reproducibility emission neutron. There was also found that the neutron production is strongly affected when conditioned for plasma focus operation insulator is exposed by a long time to air in atmospheric conditions